Open your sources.list file using your favorite text editor, e.g.
sudo nano /etc/apt/sources.list
deb http://mirrors.digitalocean.com/ubuntu/ bionic main restricted
deb http://mirrors.digitalocean.com/ubuntu/ bionic-updates main restricted
deb http://mirrors.digitalocean.com/ubuntu/ bionic universe
deb http://mirrors.digitalocean.com/ubuntu/ bionic-updates universe
deb http://mirrors.digitalocean.com/ubuntu/ bionic multiverse
deb http://mirrors.digitalocean.com/ubuntu/ bionic-updates multiverse
deb http://mirrors.digitalocean.com/ubuntu/ bionic-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse